In copper deposition processes deposition and bath properties are usually controlled and regulated by means of additives in the plating bath, including brighteners, levellers (sometimes also called boosters), and carriers (sometimes also called suppressors) (the terms “depositing” and “plating” are used interchangeably throughout the text).
Brighteners, also often called anti-suppressor additive or accelerator, are required to counter the suppressive effect of suppressor additives, which are common in such plating baths. A prominent brightener in acidic copper/copper alloy plating baths is bis-(sodium sulfopropyl)-disulfide (SPS). Typically, the total amount of brighteners in the bath during the deposition process is very low. However, such brighteners, among other additives, dramatically influence the copper deposition quality. As a result, the total amount of brighteners in a respective plating bath during the deposition process needs to be carefully monitored in order to maintain an optimal amount. This is challenging because the total amount usually continually varies due to                (i) incorporation of said brighteners into the metal deposit,        (ii) degradation of said brighteners into break down products and        (iii) formation and accumulation of electrochemically active short life-time intermediate complexes in electrochemically aged baths binding and incorporating said brighteners.        
Furthermore, the total amount of said brighteners in a plating bath is typically very low (1 to 100 ppm range) and thus, not accessible to each analytical tool.
In the art methods are described to monitor the total amount of brighteners and/or break down products in a respective plating bath.
US 2005/0247577 A1 discloses a method for measuring the concentration of an additive breakdown product in a plating bath for electrodepositing a metal. The breakdown product is 3-mercaptopropylsulfonic acid (MPS). The preferred method for monitoring is cyclic voltammetric stripping (CVS).
Choe et al. disclose in “High Accuracy Concentration Analysis of Accelerator Components in Acidic Cu Superfilling Bath”, Journal of The electrochemical Society, 163 (2) D33-D39 (2016) a modified cyclic voltammetry stripping (CVS) method to measure the concentrations of bis-(sulfopropyl) disulphide (SPS) and 3-mercapto-1-propane sulfonate (MPS) in Cu plating solutions.
EP 0 402 896 A2 relates to a method of stabilising an organic additive in an acid copper electroplating solution.
U.S. Pat. No. 6,129,830 A concerns a process to electrolytically deposit copper layers within a short preparation time, especially in the manufacture of printed circuit boards.
U.S. Pat. No. 6,508,924 B1 discloses a method for determining the quantity of additive breakdown products in an electroplating bath.
According to our own experiences, methods known in the art frequently suffer from the disadvantage that they cannot be utilized to monitor the brighteners directly or often involve numerous steps to do so. As a result they are often very time consuming. It has been also frequently observed that in particular break down products and intermediates of any kind in a plating bath (i.e. in an electrochemically aged bath) negatively affect various common analytical tools by showing detrimental effects, such that no reliable results can be obtained (for further details about the detrimental effects see the text below).